Gas diffusion anode and co2 cathode electrolyte system

ABSTRACT

A low-voltage, low-energy electrochemical system and method of removing protons and/or producing a base solution using a gas diffusion anode and a cathode electrolyte comprising dissolved carbon dioxide, while applying 2V or less across the anode and cathode.

CROSS-REFERENCE

This application is continuation-in-part of U.S. patent application Ser. No. 12/503557 filed on Jul. 16, 2009, titled: “CO2 Utilization In Electrochemical Systems”, herein incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

In many chemical processes a base solution is required to achieve a chemical reaction, e.g., to neutralize an acid, or buffer the pH of a solution, or precipitate an insoluble hydroxide and/or carbonate and/or bicarbonate from a solution. One method by which the base solution is produced is by an electrochemical system as disclosed in the above-referenced US patent application, herein incorporated by reference in its entirety. In producing a base solution electrochemically, a large amount of energy, salt and water may be used; consequently, lowering the energy and material used is highly desired.

SUMMARY OF THE INVENTION

This invention pertains to a low-voltage, low-energy electrochemical system and method of producing a base solution, utilizing a cathode electrolyte comprising dissolved carbon dioxide gas in contact with a cathode, and a gas diffusion electrode. In one embodiment, the system comprises a gas diffusion anode and a cathode in contact with a cathode electrolyte comprising dissolved carbon dioxide. In another embodiment, the method comprises applying a voltage across a gas diffusion anode and a cathode wherein the cathode contacts a cathode electrolyte comprising dissolved carbon dioxide gas. In various embodiments, the system includes a gas delivery system configured to deliver hydrogen gas to the anode; the hydrogen gas is produced at the cathode; the anode is configured to produce protons, and the cathode is configured to produce hydrogen gas and hydroxide ions on application of a voltage across the anode and the cathode; the voltage is less than 2V; a gas is not produced at the anode; the system includes a first cation exchange membrane positioned between the cathode electrolyte and a salt solution, and an anion exchange membrane positioned between the salt solution and an anode electrolyte; the anode contacts the anode electrolyte; a second cation exchange membrane is positioned between the anode and the anode electrolyte; the system is configured to migrate anions to the anode electrolyte from the salt solution through the anion exchange membrane when the voltage is applied across the anode and cathode; the system is configured to migrate chloride ions to the anode electrolyte from the salt solution through the anion exchange membrane; the system is configured to migrate cations to the cathode electrolyte from the salt solution through the first cation exchange membrane; the system is configured to migrate sodium ions to the cathode electrolyte from the salt solution through the first cation exchange membrane; the system is configured to migrate protons to the anode electrolyte from the anode; the system is configured to migrate hydroxide ions to the cathode electrolyte from the cathode; the system is configured to produce sodium hydroxide and/or sodium bicarbonate and/or sodium carbonate in the cathode electrolyte; the system is configured to produce an acid in the anode electrolyte; the system is configured to produce hydrochloric acid in the anode electrolyte; the system comprises a partition that partitions the cathode electrolyte into a first cathode electrolyte portion and a second cathode electrolyte portion, wherein the second cathode electrolyte portion contacts the cathode and comprises dissolved carbon dioxide; the first cathode electrolyte portion comprises gaseous carbon dioxide; in the system, the partition is positioned to isolate gaseous carbon dioxide gas in the first cathode electrolyte portion from cathode electrolyte in the second cathode electrolyte portion; the system is configured to produce hydroxide ions and hydrogen gas at the cathode; the system is configured to produce hydroxide ions in the cathode electrolyte; the system is configured to migrate cations to the cathode electrolyte through the first cation exchange membrane; migrate anions to the anode electrolyte through the anion exchange membrane; and migrate protons to the anode electrolyte from the anode; the system is configured to produce cations, hydroxide ions and/or carbonic acid and/or carbonate ions and/or bicarbonate ions in the cathode electrolyte; the system is configured to produce sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte; the cathode electrolyte is operatively connected to a carbon dioxide gas/liquid contactor configured to dissolve carbon dioxide in the cathode electrolyte; and the cathode electrolyte is operatively connected to a system configured to produce carbonates and/or bicarbonates and/or hydroxides from a solution comprising carbon dioxide and divalent cations.

In various embodiments, the method includes oxidizing hydrogen gas at the anode; producing protons at the anode; producing hydroxide ions and hydrogen gas at the cathode; not producing a gas at the anode; applying a voltage of 2V or less across the anode and cathode; directing hydrogen gas from the cathode to the anode; migrating protons from the anode to an anode electrolyte; interposing a cation exchange membrane between the anode and an anode electrolyte; interposing an anion exchange membrane between the anode electrolyte and a salt solution; interposing a first cation exchange membrane between the cathode electrolyte and the salt solution, and wherein the salt solution is disposed between the anion exchange membrane and the first cation exchange membrane; migrating anions from the salt solution to the anode electrolyte through the anion exchange membrane, and migrating cations from the salt solution to the cathode electrolyte through the first cation exchange membrane; producing hydroxide ions and/or carbonate ions and/or bicarbonate ions in the cathode electrolyte; and an acid in the anode electrolyte; producing sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte; and hydrochloric acid in the anode electrolyte; contacting the cathode electrolyte with a divalent cation solution comprising calcium and/or magnesium ions; and producing partially desalinated water in the salt solution.

In the system, applying a relatively low voltage across the anode and cathode, e.g., 2V or less, produces hydroxide ions and hydrogen gas at the cathode, and protons at the anode. In the system, under the applied voltage, the hydroxide ions produced at the cathode migrate into the cathode electrolyte to produce the base solution, and protons produced at the anode migrate to the anode electrolyte to produce an acid. In various embodiments, hydrogen gas produced at the cathode is directed to the anode where it is oxidized to protons. In the system, dissolving carbon dioxide in the cathode electrolyte alters the pH of the electrolyte in such a manner that the voltage required across the anode and cathode to produce the base solution is lowered. In the system, the dissolved carbon dioxide also produces carbonic acid and/or carbonate ions and/or bicarbonate ions in the cathode electrolyte, depending on the pH of the electrolyte. In various configurations, cation exchange membrane and anion exchange membranes are used in the system to separate a salt solution, e.g., a solution of sodium chloride, from the cathode electrolyte and anode electrolyte. In the system, under the applied voltage, cations in the salt solution migrate to the cathode electrolyte through the cation exchange membrane, and anions in the salt solution migrate to the anode electrolyte through an anion exchange membrane. Consequently, in the system, a base solution comprising hydroxide ions and/or dissolved carbon dioxide and/or carbonate ions and/or bicarbonate ions and/or cations from the salt solution, may be produced in the cathode electrolyte. Similarly, the anode electrolyte may produce an acid comprising protons that migrate from the anode and anions that migrate from the salt solution. In the system, a gas, e.g., chlorine or oxygen is not produced at the anode.

In various embodiments, the gas diffusion anode comprises a conductive substrate infused with a catalyst that catalyzes the oxidation of hydrogen to protons. In various embodiments, the substrate is configured such that on a first side the substrate interfaces with hydrogen, and on an opposed side the substrate interfaces with the anode electrolyte. In the system, on applying the voltages as disclosed herein across the anode and cathode, protons are produced at the substrate from oxidization of hydrogen gas. Under the applied voltages, the protons migrate to the anode electrolyte where they produce an acid.

Advantageously, with the system and method, since the voltage across the anode and cathode required to produce the hydroxide ions is lowered, the energy required to produce the base solution is lowered.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings illustrate by way of examples and not by limitation embodiments of the present system and method.

FIG. 1 is an illustration of an embodiment of the present system.

FIG. 2 is an illustration of an embodiment of the present anode system.

FIG. 3 is a flow chart of an embodiment of the present method.

FIG. 4 is an illustration of an embodiment of the present system.

FIG. 5 is an illustration of Carbonate ion/Bicarbonate ion speciation in water.

FIG. 6 is an illustration of Bicarbonate Ion generation in the cathode electrolyte.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, a salt solution comprising sodium chloride is utilized in the system to produce a base solution in the cathode electrolyte, and an acid in the anode electrolyte. In this exemplarary embodiment, sodium ions migrate from the salt solution to produce sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte; and chloride ions migrate from the salt solution to produce hydrochloric acid in the anode electrolyte. However, as can be appreciated by one ordinarily skilled in the art, since the system can be configured to utilize an equivalent salt solution, e.g., a solution of potassium sulfate and the like, to produce an equivalent result, e.g., potassium hydroxide and/or potassium carbonate and/or potassium bicarbonate in the cathode electrolyte, and sulfuric acid in the anode electrolyte, the system is not limited to using sodium chloride solution but can be configured to utilize an equivalent salt solution. Therefore, to the extent that equivalent salts can be used, these equivalents are within the scope of the appended claims.

As disclosed in U.S. patent application Ser. No. 12/503557 filed on Jul. 16, 2009, titled: “CO2 Utilization In Electrochemical Systems”, herein incorporated by reference in its entirety, in various embodiments, carbon dioxide is absorbed into the cathode electrolyte utilizing a gas mixer/gas absorber. In one embodiment, the gas mixer/gas absorber comprises a series of spray nozzles that produces a flat sheet or curtain of liquid into which the gas is absorbed; in another embodiment, the gas mixer/gas absorber comprises a spray absorber that creates a mist and into which the gas is absorbed; in other embodiments, other commercially available gas/liquid absorber, e.g., an absorber available from Neumann Systems, Colorado, USA is used.

The carbon dioxide used in the system is obtained from various sources including carbon dioxide from combustion gases of fossil fuelled electrical power generating plants, cement plants, ore processing plants and the like. In some embodiments, the carbon dioxide may comprise other gases, e.g., nitrogen, oxides of nitrogen (nitrous oxide, nitric oxide), sulfur and sulfur gases (sulfur dioxide, hydrogen sulfide), and vaporized materials. In some embodiments, the system includes a gas treatment system that removes constituents in the carbon dioxide gas stream before the gas is utilized in the cathode electrolyte. In some embodiments, a portion of, or the entire amount of, cathode electrolyte comprising bicarbonate ions and/or carbonate ions/and or hydroxide ions is withdrawn from the system and is contacted with carbon dioxide gas in an exogenous carbon dioxide gas/liquid contactor to increase the absorbed carbon dioxide content in the solution. In some embodiments, the solution enriched with carbon dioxide is returned to the cathode compartment; in other embodiments, the solution enriched with carbon dioxide is reacted with a solution comprising divalent cations to produce divalent cation hydroxides, carbonates and/or bicarbonates. In some embodiments, the pH of the cathode electrolyte is adjusted upwards by hydroxide ions that migrate from the cathode, and/or downwards by dissolving carbon dioxide gas in the cathode electrolyte to produce carbonic acid and carbonic ions that react with and remove hydroxide ions. Thus it can be appreciated that the pH of the cathode electrolyte is determined, at least in part, by the balance of these two processes.

Referring to FIG. 1 herein, the system 100 in one embodiment comprises a gas diffusion anode 102 and a cathode 106 in contact with a cathode electrolyte 108, 108A, 108B comprising dissolved carbon dioxide 107A. The system in various embodiments includes a gas delivery system 112 configured to deliver hydrogen gas to the anode 102; in some embodiments, the hydrogen gas is obtained from the cathode 106. In the system, the anode 102 is configured to produce protons, and the cathode 106 is configured to produce hydroxide ions and hydrogen gas when a low voltage 114, e.g., less than 2V is applied across the anode and the cathode. In the system, a gas is not produced at the anode 102.

In the system as illustrated in FIG. 1, first cation exchange membrane 116 is positioned between the cathode electrolyte 108, 108A, 108B and a salt solution 118; and an anion exchange membrane 120 is positioned between the salt solution 118 and the anode electrolyte 104 in a configuration where the anode electrolyte 104 is separated from the anode 102 by second cation exchange membrane 122. In the system, the second cation exchange membrane 122 is positioned between the anode 102 and the anode electrolyte 104 such that anions may migrate from the salt solution 118 to the anode electrolyte 104 through the anion exchange membrane 120; however, anions are prevented from contacting the anode 102 by the second cation exchange membrane 122 adjacent to the anode 102.

In various embodiments, the system is configurable to migrate anions, e.g., chloride ions, from the salt solution 118 to the anode electrolyte 104 through the anion exchange membrane 120; migrate cations, e.g., sodium ions from the salt solution 118 to the cathode electrolyte 108, 108A, 108B through the first cation exchange membrane 116; migrate protons from the anode 102 to the anode electrolyte 104; and migrate hydroxide ions from the cathode 106 to the cathode electrolyte 108, 108A, 108B. Thus, in various embodiments, the system can be configured to produce sodium hydroxide and/or sodium bicarbonate and/or sodium carbonate in the cathode electrolyte 108, 108A, 108B; and produce an acid e.g., hydrochloric acid 124 in the anode electrolyte.

In various embodiments as illustrated in FIG. 1, the system comprises a partition 126 that partitions the cathode electrolyte 108 into a first cathode electrolyte portion 108A and a second cathode electrolyte portion 108B, wherein the second cathode electrolyte portion 108B, comprising dissolved carbon dioxide, contacts the cathode 106; and wherein the first cathode electrolyte portion 108A comprising dissolved carbon dioxide and gaseous carbon dioxide is in contact with the second cathode electrolyte portion 108B under the partition 126. In the system, the partition is positioned in the cathode electrolyte such that gaseous carbon dioxide in the first cathode electrolyte portion 108A is isolated from cathode electrolyte in the second cathode electrolyte portion 108B.

Thus, as can be appreciated, in various embodiments, on applying the present voltage across the anode and cathode, the system can be configured to produce hydroxide ions and hydrogen gas at the cathode 106; migrate hydroxide ions from the cathode into the cathode electrolyte 108, 108B, 108A; migrate cations from the salt solution 118 to the cathode electrolyte through the first cation exchange membrane 116; migrate chloride ions from the salt solution 118 to the anode electrolyte 104 through the anion exchange membrane 120; and migrate protons from the anode 102 to the anode electrolyte 104. Hence, depending on the salt solution 118 used, the system can be configured to produce a base solution, e.g., sodium hydroxide in the cathode electrolyte.

In some embodiments, the system is operatively connected to a carbon dioxide gas/liquid contactor 128 configured to remove cathode electrolyte from the system and dissolve carbon dioxide in the cathode electrolyte in the gas/liquid contactor before the cathode electrolyte is returned to the system.

In other embodiments, the cathode electrolyte is operatively connected to a system (not shown) that is configured to precipitate divalent cation carbonates and/or divalent cation bicarbonates and/or divalent cation hydroxides from a solution comprising carbon dioxide gas and divalent cations.

FIG. 2 illustrates a schematic of a suitable gas diffusion anode that can be used in the system. In various embodiments, the gas diffusion anode comprises a conductive substrate 130 infused with a catalyst 136 that is capable of catalyzing the oxidation of hydrogen gas to protons when the present voltages are applied across the anode and cathode. In some embodiments, the anode comprises a first side that interfaces with hydrogen gas provided to the anode, and an opposed second side 134 that interfaces with the anode electrolyte 104. In some embodiments, the portion of the substrate 132 that interfaces with the hydrogen gas is hydrophobic and is relatively dry; and the portion of the substrate 134 that interfaces with the anode electrolyte 104 is hydrophilic and may be wet, which facilitates migration of protons from the anode to the anode electrolyte. Preferably, the substrate 130 may be selected such that an appropriate side is hydrophilic or hydrophobic as described herein, as well as for a low ohmic resistance for electron conduction from the anode, and good porosity for proton migration to the anode electrolyte 116. In various embodiments, the catalyst may comprise platinum, ruthenium, iridium, rhodium, manganese, silver or alloys thereof. Suitable gas diffusion anodes are available commercially, e.g., from E-TEK (USA) and other suppliers

As is illustrated in FIG. 1, the system includes a salt solution 118 located between the anode electrolyte 104 and the cathode electrolyte 108, 108A, 108B. In various embodiments, the cathode electrolyte is separated from the salt solution by a first cation exchange membrane 116 that is allows migration of cations, e.g., sodium ions, from the salt solution to the cathode electrolyte. The first cation exchange membrane 116 is also capable of blocking the migration of anions from the cathode electrolyte 108, 108A, 108B to the salt solution 118. In various embodiments, the anode electrolyte 104 is separated from the salt solution 118 by an anion exchange membrane 108 that will allow migration of anions, e.g., chloride ions, from the salt solution 118 to the anode electrolyte 104. The anion exchange membrane, however, will block the migration of cations, e.g., protons from the anode electrolyte 104 to the salt solution 118.

With reference to FIGS. 1 and 2, the system includes a hydrogen gas supply system 112 configured to provide hydrogen gas to the anode 102. The hydrogen may be obtained from the cathode 106 or may be obtained from external source, e.g., from a commercial hydrogen gas supplier, e.g., at start-up of the system when the hydrogen supply from the cathode is insufficient. In the system, the hydrogen gas is oxidized to protons and electrons; un-reacted hydrogen gas is recovered and circulated 140 at the anode.

Referring to FIG. 1, in operation, the cathode electrolyte 108, 108A, 108B is initially charged with a base electrolyte, e.g., sodium hydroxide solution, and the anode electrolyte 104 is initially charged with an acidic electrolyte, e.g., dilute hydrochloric acid. The cathode electrolyte is also initially charged with carbon dioxide gas 107A, 128, and hydrogen gas is provided to the anode. In the system, on applying a voltage across the anode and cathode, protons produced at the anode will enter into the anode electrolyte and attempt to migrate from the anode electrolyte 104 to the cathode 106 via the salt solution 118 between the cathode and anode. However, since the anion exchange membrane will block the migration of protons to the salt solution, the protons will accumulate in the anode electrolyte 104.

Simultaneously at the cathode 106, the voltage across the anode and cathode will produce hydroxide ions and hydrogen gas at the cathode. In some embodiments, the hydrogen produced at the cathode is recovered and directed to the anode 102 where it is oxidized to protons. In the system, hydroxide ions produced at the cathode 106 will enter into the cathode electrolyte 108, 108A, 108B from where they will attempt to migrate to the anode 102 via the salt solution 118 between the cathode and anode. However, since the cathode electrolyte 108, 108A, 108B is separated from the salt solution electrolyte by the first cation exchange membrane 116 which will block the passage of anions, the first cation exchange membrane will block the migration of hydroxide ions from the cathode electrolyte to the salt solution; consequently, the hydroxide ions will accumulate in the cathode electrolyte 108, 108A, 108B.

In the system as illustrated in FIG. 1, with the voltage across the anode and cathode, since the salt solution is separated from the cathode electrolyte by the first cation exchange membrane 116, cations in the salt solution, e.g., sodium ions, will migrate through the first cation exchange membrane 116 to the cathode electrolyte 108, 108A, 108B, and anions, e.g., chloride ions, will migrate to the anode electrolyte through the anion exchange membrane 120. Consequently, in the system, as illustrated in FIG. 1, an acid, e.g., hydrochloric acid 124 will be produced in the anode electrolyte 104, and base solution, e.g., sodium hydroxide will be produced in the cathode electrolyte. As can be appreciated, with the migration of cations and anions from the salt solution, the system in some embodiments can be configured to produce a partly de-ionized salt solution from the salt solution 118. In various embodiments, this partially de-ionized salt solution can be used as feed-water to a desalination facility (not shown) where it can be further processed to produce desalinated water as described in commonly assigned U.S. patent application Ser. No. 12/163,205 filed on Jun. 27, 2008, herein incorporated by reference in its entirety; alternatively, the solution can be used in industrial and agricultural applications where its salinity is acceptable.

With reference to FIG. 1, the system in some embodiments includes a second cation exchange membrane 124, attached to the anode substrate 105, such that it separates the anode 102 from the anode electrolyte. In this configuration, as the second cation exchange membrane 122 is permeable to cations, protons formed at the anode will migrate to the anode electrolyte as described herein; however, as the second cation exchange membrane 122 is impermeable to anions, anions, e.g., chloride ions, in the anode electrolyte will be blocked from migrating to the anode 102, thereby avoiding interaction between the anode and the anions that may interact with the anode, e.g., by corrosion.

With reference to FIG. 1, in some embodiments, the system includes a partition 128 configured into J-shape structure and positioned in the cathode electrolyte 108, 108A, 108B to define an upward-tapering channel 144 in the upper portion of the cathode electrolyte compartment. The partition also defines a downward-tapering channel 146 in lower portion of the cathode electrolyte. Thus, with the partition in the place, the cathode electrolyte 108 is partitioned into the first cathode electrolyte portion 108A and a second cathode electrolyte portion 108B. As is illustrated in FIG. 1, cathode electrolyte in the first cathode electrolyte portion 108A is in contact with cathode electrolyte in the second cathode electrolyte portion 108B; however, a gas in the first electrolyte portion 108A, e.g., carbon dioxide, is prevented from mixing with cathode electrolyte in the second cathode electrolyte 108B.

With reference to FIG. 1, the system in various embodiments includes a cathode electrolyte circulating system 142 adapted for withdrawing and circulating cathode electrolyte in the system. In one embodiment, the cathode electrolyte circulating system comprises a carbon dioxide gas/liquid contactor 128 that is adapted for dissolving carbon dioxide in the circulating cathode electrolyte, and for circulating the electrolyte in the system. As can be appreciated, since the pH of the cathode electrolyte can be adjusted by withdrawing and/or circulating cathode electrolyte from the system, the pH of the cathode electrolyte compartment can be by regulated by regulating an amount of cathode electrolyte removed from the system through the carbon dioxide gas/liquid contactor 128.

In an alternative as illustrated in FIG. 4, the system comprises a cathode 106 in contact with a cathode electrolyte 108 and an anode 102 in contact with an anode electrolyte 104. In this system, the cathode electrolyte comprises a salt solution that functions as the cathode electrolyte as well as a source of chloride and sodium ions for the base and acid solution produced in the system. In this system, the cathode electrolyte is separated from the anode electrolyte by an anion exchange membrane 120 that allows migration of anions, e.g., chloride ions, from the salt solution to the anode electrolyte. As is illustrated in FIG. 4, the system includes a hydrogen gas delivery system 112 configured to provide hydrogen gas to the anode. The hydrogen may be obtained from the cathode and/or obtained from an external source, e.g., a commercial hydrogen gas supplier e.g., at start-up of operations when the hydrogen supply from the cathode is insufficient. In various embodiments, the hydrogen delivery system is configured to deliver gas to the anode where oxidation of the gas is catalyzed to protons and electrons. In some embodiments, un-reacted hydrogen gas in the system is recovered and re-circulated to the anode.

Referring to FIG. 4, as with the system of FIG. 1, on applying a voltage across the anode and cathode, protons produced at the anode from oxidation of hydrogen will enter into the anode electrolyte from where they will attempt to migrate to the cathode electrolyte across the anion exchange membrane 120. However, since the anion exchange membrane 120 will block the passage of cations, the protons will accumulate in the anode electrolyte. At the same time, however, the anion exchange membrane 120 being pervious to anions will allow the migration of anions, e.g., chloride ions from the cathode electrolyte to the anode, thus in this embodiment, chloride ions will migrate to the anode electrolyte to produce hydrochloric acid in the anode electrolyte. In this system, the voltage across the anode and cathode is adjusted to a level such that hydroxide ions and hydrogen gas are produced at the cathode without producing a gas, e.g., chlorine or oxygen, at the anode. In this system, since cations will not migrate from the cathode electrolyte across the anion exchange membrane 116, sodium ions will accumulate in the cathode electrolyte 108 to produce a base solution with hydroxide ions produced at the cathode. In embodiments where carbon dioxide gas is dissolved in the cathode electrolyte, sodium ions may also produce sodium bicarbonate and or sodium carbonate in the cathode electrolyte as described herein with reference to FIG. 1.

With reference to FIG. 1, depending on the pH of the cathode electrolyte, carbon dioxide gas introduced into the first cathode electrolyte portion 108A will dissolve in the cathode electrolyte and reversibly dissociate and equilibrate to produce carbonic acid, protons, carbonate and/or bicarbonate ions in the first cathode electrolyte compartment as follows:

CO₂+H₂O<==>H₂CO₃<==>H⁺+HCO₃ ⁻<==>H⁺+CO₃ ²⁻

In the system, as cathode electrolyte in the first cathode electrolyte portion 108A may mix with second cathode electrolyte portion 108B, the carbonic acid, bicarbonate and carbonate ions formed in the first cathode electrolyte portion 108A by absorption of carbon dioxide in the cathode electrolyte may migrate and equilibrate with cathode electrolyte in the second cathode electrolyte portion 108B. Thus, in various embodiments, first cathode electrolyte portion 108A may comprise dissolved and un-dissolved carbon dioxide gas, and/or carbonic acid, and/ or bicarbonate ions and/or carbonate ions; while second cathode electrolyte portion 108B may comprise dissolved carbon dioxide, and/or carbonic acid, and/ or bicarbonate ions and/or carbonate ions.

With reference to FIG. 1, on applying a voltage across anode 102 and cathode 108, the system 100 may produce hydroxide ions and hydrogen gas at the cathode from water, as follows:

2H₂O+2e⁻=H₂+2OH⁻

As cathode electrolyte in first cathode electrolyte portion 108A can intermix with cathode electrolyte in second cathode electrolyte portion 108B, hydroxide ions formed in the second cathode electrolyte portion may migrate and equilibrate with carbonate and bicarbonate ions in the first cathode electrolyte portion 108A. Thus, in various embodiments, the cathode electrolyte in the system may comprise hydroxide ions and dissolved and/or un-dissolved carbon dioxide gas, and/or carbonic acid, and/ or bicarbonate ions and/or carbonate ions. In the system, as the solubility of carbon dioxide and the concentration of bicarbonate and carbonate ions in the cathode electrolyte are dependent on the pH of the electrolyte, the overall reaction in the cathode electrolyte 104 is either:

2H₂O+2CO₂+2e⁻=H₂+2HCO₃ ⁻  Scenario 1

or

H₂O+CO₂+2e⁻=H₂+CO₃ ²⁻  Scenario 2

or a combination of both, depending on the pH of the cathode electrolyte. This is illustrated in the carbonate speciation diagram of FIG. 5:

For either scenario, the overall cell potential of the system can be determined through the Gibbs energy change of the reaction by the formula:

E _(cell) =ΔG/nF

Or, at standard temperature and pressure conditions:

E° _(cell) =−ΔG°/nF

where, E_(cell) is the cell voltage, ΔG is the Gibbs energy of reaction, n is the number of electrons transferred, and F is the Faraday constant (96485 JNmol). The E_(cell) of each of these reactions is pH dependent based on the Nernst equestion as demonstrated in FIG. 6 for Scenario 1, as discussed below.

Also, for either scenario, the overall cell potential can be determined through the combination of Nernst equations for each half cell reaction:

E=E°−RT In(Q)/n F

where, E° is the standard reduction potential, R is the universal gas constant, (8.314 J/mol K) T is the absolute temperature, n is the number of electrons involved in the half cell reaction, F is Faraday's constant (96485 JN mol), and Q is the reaction quotient such that:

E _(total)=E _(cathode)+E _(anode).

When hydrogen is oxidized to protons at the anode as follows:

H₂=2H⁺+2e⁻,

E° is 0.00 V, n is 2, and Q is the square of the activity of H⁻so that:

E _(anode)=+0.059pH _(a),

where pH_(a) is the pH of the anode electrolyte. When water is reduced to hydroxide ions and hydrogen gas at the cathode as follows:

2H₂O+2e⁻=H₂+2OH⁻,

E° is −0.83 V, n is 2, and Q is the square of the activity of OH⁻so that:

E _(cathode)=−0.059pH _(C),

where pH_(C) is the pH of the cathode electrolyte.

For either Scenario, the E for the cathode and anode reactions varies with the pH of the anode and cathode electrolytes. Thus, for Scenario 1 if the anode reaction, which is occurring in an acidic environment, is at a pH of 0, then the E of the reaction is 0V for the half cell reaction. For the cathode reaction, if the generation of bicarbonate ions occur at a pH of 7, then the theoretical E is 7×(−0.059 V)=−0.413V for the half cell reaction where a negative E means energy is needed to be input into the half cell or full cell for the reaction to proceed. Thus, if the anode pH is 0 and the cathode pH is 7 then the overall cell potential would be −0.413V, where:

E _(total)=−0.059 (pH _(a) −pH _(C))=−0.059ΔpH.

For Scenario 2 in which carbonate ions are produced, if the anode pH is 0 and the cathode pH is 10, this would represent an E of 0.59 V.

Thus, in various embodiments, directing CO₂ gas into the cathode electrolyte may lower the pH of the cathode electrolyte by producing bicarbonate ions and/or carbonate ions in the cathode electrolyte, which consequently may lower the voltage across the anode and cathode in producing hydroxide, carbonate and/or bicarbonate in the cathode electrolyte.

Thus, as can be appreciated, if the cathode electrolyte is allowed to increase to a pH of 14 or greater, the difference between the anode half-cell potential (represented as the thin dashed horizontal line, Scenario 1, above) and the cathode half cell potential (represented as the thick solid sloping line in Scenario 1, above) will increase to 0.83V. With increased duration of cell operation without CO₂ addition or other intervention, e.g., diluting with water, the required cell potential will continue to increase. The cell potential may also increase due to ohmic resistance loses across the membranes in the electrolyte and the cell's overvoltage potential.

Herein, an overvoltage potential refers to the voltage difference between a thermodynamically determined half-cell reduction potential, and the experimentally observed potential at which the redox reaction occurs. The term is related to a cell voltage efficiency as the overvoltage potential requires more energy than is thermodynamically required to drive a reaction. In each case, the extra energy is lost as heat. Overvoltage potential is specific to each cell design and will vary between cells and operational conditions even for the same reaction.

In embodiments wherein it is desired to produce bicarbonate and/or carbonate ions in the cathode electrolyte, the system as illustrated in FIGS. 1-2, and as described above with reference to production of hydroxide ions in the cathode electrolyte, can be configured to produce bicarbonate ions and/or carbonate ions in the first cathode electrolyte by dissolving carbon dioxide in the first cathode electrolyte and applying a voltage of less than 3V, or less than 2.5 V, or less than 2V, or less than 1.5V such as less than 1.0V, or even less than 0.8 V or 0.6V across the cathode and anode.

In various embodiments, hydroxide ions, carbonate ions and/or bicarbonate ions produced in the cathode electrolyte, and hydrochloric acid produced in the anode electrolyte are removed from the system, while sodium chloride in the salt solution electrolyte is replenished to maintain continuous operation of the system. As can be appreciated, in various embodiments, the system can be configured to operate in various production modes including batch mode, semi-batch mode, continuous flow mode, with or without the option to withdraw portions of the hydroxide solution produced in the cathode electrolyte, or withdraw all or a portions of the acid produced in the anode electrolyte, or direct the hydrogen gas produced at the cathode to the anode where it may be oxidized.

In various embodiments, hydroxide ions, bicarbonate ions and/or carbonate ion solutions are produced in the cathode electrolyte when the voltage applied across the anode and cathode is less than 3V, 2.9V or less, 2.8V or less, 2.7V or less, 2.6V or less, 2.5V or less, 2.4V or less, 2.3V or less, 2.2V or less, 2.1V or less, 2.0V or less, 1.9V or less, 1.8V or less, 1.7V or less, 1.6V, or less 1.5V or less, 1.4V or less, 1.3V or less, 1.2V or less, 1.1V or less, 1.0V or less, 0.9V or less or less, 0.8V or less, 0.7V or less, 0.6V or less, 0.5V or less, 0.4V or less, 0.3V or less, 0.2V or less, or 0.1 V or less.

In another embodiment, the voltage across the anode and cathode can be adjusted such that gas will form at the anode, e.g., oxygen or chlorine, while hydroxide ions, carbonate ions and bicarbonate ions are produced in the cathode electrolyte and hydrogen gas is generated at the cathode. However, in this embodiment, hydrogen gas is not supplied to the anode. As can be appreciated by one ordinarily skilled in the art, in this embodiment, the voltage across the anode and cathode will be higher compared to the embodiment when a gas does not form at the anode.

With reference to FIGS. 1-2, in various embodiments, the invention provides for a system comprising one or more anion exchange membrane 120, and cation exchange membranes 116, 122 located between the gas diffusion anode 102 and the cathode 106. In various embodiments, the membranes should be selected such that they can function in an acidic and/or basic electrolytic solution as appropriate. Other desirable characteristics of the membranes include high ion selectivity, low ionic resistance, high burst strength, and high stability in an acidic electrolytic solution in a temperature range of 0° C. to 100° C. or higher, or a base solution in similar temperature range may be used. In some embodiments, a membrane that is stable in the range of 0° C. to 80° C., or 0° C. to 90° C., but not stable above these ranges may be used. For other embodiments, it may be useful to utilize an ion-specific ion exchange membranes that allows migration of one type of cation but not another; or migration of one type of anion and not another, to achieve a desired product or products in an electrolyte. In some embodiments, the membrane should be stable and functional for a desirable length of time in the system, e.g., several days, weeks or months or years at temperatures in the range of 0° C. to 80° C., or 0° C. to 90° C. and higher and/or lower. In some embodiments, for example, the membranes should be stable and functional for at least 5 days, 10 days, 15 days, 20 days, 100 days, 1000 days or more in electrolyte temperatures at 80° C., 70° C., 60° C., 50° C., 40° C., 30° C., 20° C., 10° C., 5° C. and more or less.

As can be appreciated, the ohmic resistance of the membranes will affect the voltage drop across the anode and cathode, e.g., as the ohmic resistance of the membranes increase, the voltage drop across the anode and cathode will increase, and vice versa. Membranes currently available can be used and they include membranes with relatively ohmic resistance and relatively high ionic mobility; similarly, membranes currently available with relatively high hydration characteristics that increases with temperatures, and thus decreasing the ohmic resistance can be used. Consequently, as can be appreciated, by selecting currently available membranes with lower ohmic resistance, the voltage drop across the anode and cathode at a specified temperature can be lowered.

Scattered through currently available membrane are ionic channels consisting of acid groups. These ionic channels may extend from the internal surface of the matrix to the external surface and the acid groups may readily bind water in a reversible reaction as water-of-hydration. This binding of water as water-of-hydration follows first order reaction kinetics, such that the rate of reaction is proportional to temperature. Consequently, currently available membranes can be selected to provide a relatively low ohmic and ionic resistance while providing for improved strength and resistance in the system for a range of operating temperatures. Suitable membranes are commercially available from Asahi Kasei of Tokyo, Japan; or from Membrane International of Glen Rock, N.J., and USA.

In various embodiments, the cathode electrolyte 108, 108A, 108B is operatively connected to a waste gas treatment system (not illustrated) where the base solution produced in the cathode electrolyte is utilized, e.g., to sequester carbon dioxide contained in the waste gas by contacting the waste gas and the cathode electrolyte with a solution of divalent cations to precipitate hydroxides, carbonates and/or bicarbonates as described in commonly assigned U.S. patent application Ser. No. 12/344,019 filed on Dec. 24, 2008, herein incorporated by reference in its entirety. The precipitates, comprising, e.g., calcium and magnesium hydroxides, carbonates and bicarbonates in various embodiments may be utilized as building materials, e.g., as cements and aggregates, as described in commonly assigned U.S. patent application Ser. No. 12/126,776 filed on May 23, 2008, supra, herein incorporated by reference in its entirety. In some embodiments, some or all of the carbonates and/or bicarbonates are allowed to remain in an aqueous medium, e.g., a slurry or a suspension, and are disposed of in an aqueous medium, e.g., in the ocean depths or a subterranean site.

In various embodiments, the cathode and anode are also operatively connected to an off-peak electrical power-supply system 114 that supplies off-peak voltage to the electrodes. Since the cost of off-peak power is lower than the cost of power supplied during peak power-supply times, the system can utilize off-peak power to produce a base solution in the cathode electrolyte at a relatively lower cost.

In another embodiment, the system produces an acid, e.g., hydrochloric acid 124 in the anode electrolyte 104. In various embodiments, the anode compartment is operably connected to a system for dissolving minerals and/or waste materials comprising divalent cations to produce a solution of divalent cations, e.g., Ca++ and Mg++. In various embodiments, the divalent cation solution is utilized to precipitate hydroxides, carbonates and/or bicarbonates by contacting the divalent cation solution with the present base solution and a source of carbon dioxide gas as described in U.S. patent application Ser. No. 12/344,019 filed on Dec. 24, 2008, supra, herein incorporated by reference in its entirety. In various embodiments, the precipitates are used as building materials e.g., cement and aggregates as described in commonly assigned U.S. patent application Ser. No. 12/126,776, supra, herein incorporated by reference in its entirety.

With reference to FIG. 1, on applying a voltage across the anode 102 and cathode 106, protons will form at the anode from oxidation of hydrogen gas supplied to the anode, while hydroxide ions and hydrogen gas will form at the cathode electrolyte from the reduction of water, as follows:

H₂=2H⁺+2e⁻  (anode, oxidation reaction)

2H₂O+2e⁻=H₂+2OH⁻  (cathode, reduction reaction)

Since protons are formed at the anode from hydrogen gas provided to the anode; and since a gas such as oxygen does not form at the anode; and since water in the cathode electrolyte forms hydroxide ions and hydrogen gas at the cathode, the system will produce hydroxide ions in the cathode electrolyte and protons in the anode electrolyte when a voltage is applied across the anode and cathode. Further, as can be appreciated, in the present system since a gas does not form at the anode, the system will produce hydroxide ions in the cathode electrolyte and hydrogen gas at the cathode and hydrogen ions at the anode when less than 2V is applied across the anode and cathode, in contrast to the higher voltage that is required when a gas is generated at the anode, e.g., chlorine or oxygen. For example, in various embodiments, hydroxide ions are produced when less than 2.0V, 1.5V, 1.4V, 1.3V, 1.2V, 1.1V, 1.0V, 0.9V, 0.8V, 0.7V, 0.6V, 0.5V, 0.4V, 0.3V, 0.2V, 0.1 V or less is applied across the anode and cathode.

As discussed above, in the system, on applying a voltage across the anode 102 and cathode 106, the positively charged protons formed at the anode will attempt to migrate to the cathode through the anode electrolyte 104, while the negatively charged hydroxide ions formed at the cathode will attempt to migrate to the anode through the cathode electrolyte 108, 108A, 108B. As is illustrated in FIG. 1 and with reference to hydroxide ions in the cathode electrolyte 108, 108A, 108B, since the first cation exchange membrane 116 will restrict the migration of anions from the cathode electrolyte 108, 108A, 108B, and since the anion exchange membrane 120 will prevent the migration of anions from the anode electrolyte 104 to the salt solution 118, the hydroxide ions generated in the cathode electrolyte will be prevented from migrating out of the cathode electrolyte through the cation exchange membrane. Consequently, on applying the voltage across the anode and cathode, the hydroxide ions produced at the cathode will be contained in the cathode electrolyte. Thus, depending on the flow rate of fluids into and out of the cathode electrolyte and the rate of carbon dioxide dissolution in the cathode electrolyte, the pH of the cathode electrolyte will adjust, e.g., the pH may increase, decrease or remain the same.

In various embodiments, depending on the ionic species desired in cathode electrolyte 108, 108A, 108B and/or the anode electolyte 104 and/or the salt solution 118, alternative reactants can be utilized. Thus, for example, if a potassium salt such as potassium hydroxide or potassium carbonate is desired in the cathode electrolyte 1108, 108A, 108B, then a potassium salt such as potassium chloride can be utilized in the salt solution 118. Similarly, if sulfuric acid is desired in the anode electrolyte, then a sulfate such as sodium sulfate can be utilized in the salt solution 118. As described in various embodiments herein, carbon dioxide gas is absorbed in the cathode electrolyte; however, it will be appreciated that other gases, including volatile vapors, can be absorbed in the electrolyte, e.g., sulfur dioxide, or organic vapors to produce a desired result. As can be appreciated, the gas can be added to the electrolyte in various ways, e.g., by bubbling it directly into the electrolyte, or dissolving the gas in a separate compartment connected to the cathode compartment and then directed to the cathode electrolyte as described herein.

With reference to FIGS. 1 and 3, method 300 in various embodiments comprises a step 302 of applying a voltage across a cathode 106 and a gas diffusion anode 102 in an electrochemical system 100, wherein the cathode contacts a cathode electrolyte comprising dissolved carbon dioxide. In some embodiments, the method includes a step of providing hydrogen to the gas diffusion anode 102; a step of contacting the cathode 106 with a cathode electrolyte 108, 108A, 108B comprising dissolved carbon dioxide gas 107A; and a step of applying a voltage 114 across the anode and cathode; a step whereby protons are produced at the anode and hydroxide ions and hydrogen gas produced at the cathode; a step whereby a gas is not produced at the anode when the voltage is applied across the anode and cathode; a step wherein the voltage applied across the anode and cathode is less than 2V; a step comprising directing hydrogen gas from the cathode to the anode; a step comprising whereby protons are migrated from the anode to an anode electrolyte; a step comprising interposing an anion exchange membrane between the anode electrolyte and the salt solution; a step comprising interposing a first cation exchange membrane between the cathode electrolyte and the salt solution, wherein the salt solution is contained between the anion exchange membrane and the first cation exchange membrane; a step comprising whereby anions migrate from the salt solution to the anode electrolyte through the anion exchange membrane, and cations migrate from the salt solution to the cathode electrolyte through the first cation exchange membrane; a step comprising producing hydroxide ions and/or carbonate ions and/or bicarbonate ions in the cathode electrolyte; a step comprising producing an acid in the anode electrolyte; a step comprising producing sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte; a step whereby hydrochloric acid is produced in the anode electrolyte; a step comprising contacting the cathode electrolyte with a divalent cation solution, wherein the divalent cations comprise calcium and magnesium ions; a step comprising producing partially desalinated water from the salt solution; a step comprising withdrawing a first portion of the cathode electrolyte and contacting the first portion of cathode electrolyte with carbon dioxide; and a step comprising contacting the first portion of cathode electrolyte with a divalent cation solution.

In various embodiments, hydroxide ions are formed at the cathode 106 and in the cathode electrolyte 108, 108A, 108B by applying a voltage of less than 2V across the anode and cathode without forming a gas at the anode, while providing hydrogen gas at the anode for oxidation at the anode. In various embodiments, method 300 does not form a gas at the anode when the voltage applied across the anode and cathode is less than 3V or less, 2.9V or less, 2.8V or less, 2.7V or less, 2.6V or less, 2.5V or less, 2.4V or less, 2.3V or less, 2.2V or less, 2.1V or less, 2.0V or less, 1.9V or less, 1.8V or less, 1.7V or less, 1.6V or less, 1.5V or less, 1.4V or less, 1.3V or less, 1.2V or less, 1.1V or less, 1.0V or less, 0.9V or less, 0.8V or less, 0.7V or less, 0.6V or less, 0.5V or less, 0.4V or less, 0.3V or less, 0.2V or less, or 0.1 V or less, while hydrogen gas is provided to the anode where it is oxidized to protons. As will be appreciated by one ordinarily skilled in the art, by not forming a gas at the anode and by providing hydrogen gas to the anode for oxidation at the anode, and by otherwise controlling the resistance in the system for example by decreasing the electrolyte path lengths and by selecting ionic membranes with low resistance and any other method know in the art, hydroxide ions can be produced in the cathode electrolyte with the present lower voltages.

In various embodiments, hydroxide ions, bicarbonate ions and carbonate ions are produced in the cathode electrolyte where the voltage applied across the anode and cathode is less than 3.0V, 2.9V, 2.8V, 2.7V, 2.6V, 2.5V, 2.4V, 2.3V, 2.2V, 2.1V, 2.0V, 1.9V, 1.8V, 1.7V, 1.6V, 1.5V, 1.4V, 1.3V, 1.2V, 1.1V, 1.0V, 0.9V, 0.8V, 0.7V, 0.6V, 0.5V, 0.4V, 0.3V, 0.2V, 0.1V or less without forming a gas at the anode. In various embodiments, the method is adapted to withdraw and replenish at least a portion of the cathode electrolyte and the acid in the anode electrolyte back into the system in either a batch, semi-batch or continuous mode of operation.

In an exemplarary embodiment, a system configured substantially as illustrated in FIGS. 1 and 2 was operated with a constant current density applied across the electrodes at steady state conditions while carbon dioxide gas was continuously dissolved into the cathode electrolyte, at various temperatures and voltages. In the system, a platinum catalyst, gas diffusion anode obtained from E-TEK Corporation, (USA) was used as the anode. A Raney nickel deposited onto a nickel gauze substrate was used as the cathode. In the system, the initial acid concentration in the anode electrolyte was 1 M; the initial sodium chloride salt solution was 5 M; and the initial concentration of the sodium hydroxide solution in the cathode compartment was 1 M. In the system, the pH of the cathode compartment was maintained at either 8 or 10 by regulating the amount of carbon dioxide dissolved in the cathode electrolyte.

TABLE 1 Experimental Current Density, Temperature and Voltage Characteristics of the System T Potential Current density (° C.) (V) pH (mA/cm²) 25 0.8 10 8.6 8 11.2 1.2 10 28.3 8 29.2 1.6 10 50.2 8 50.6 75 0.8 10 13.3 8 17.8 1.2 10 45.3 8 49.8 1.6 10 80.8 8 84.7

As is illustrated in Table 1, a range of current densities was achieved across the electrode in the system. As will be appreciated by one ordinarily skilled in the art, the current density that can be achieved with other configurations of the system may vary, depending on several factors including the cumulative electrical resistance losses in the cell, environmental test conditions, the over-potential associated with the anodic and cathodic reactions, and other factors.

It will also be appreciated that the current densities achieved in the present configuration and as set forth in Table 1 are correlated with the production of hydroxide ions at the cathode, and thus are correlated with the production of sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte, as follows. With reference to Table 1, at 75° C., 0.8 V and a pH of 10, each cm² of electrode passed 13.3 mA of current, where current is a measure of charge passed (Coulomb) per time (second). Based on Faraday's Laws, the amount of product, e.g., hydroxide ions, produced at an electrode is proportional to the total electrical charge passed through the electrode as follows:

n=(I*t)/(F*z)

where n is moles of product, I is a current, t is time, F is Faraday's constant, and z is the electrons transferred per product ionic species (or reagent ionic species). Thus, based on the present example, 1.38×10⁻⁴ moles of hydroxide ions are produced per second per cm² of electrode, which is correlated with the production of sodium hydroxide in the cathode electrolyte. In the system the production rate of NaOH dictates the production rate of NaHCO₃ and Na₂CO₃ through Le Chatelier's principle following the net chemical equilibria equations of

H₂CO₃+OH⁻=H₂O+HCO₃ ⁻

and

HCO₃ ⁻+OH⁻=H₂O+CO₃ ²⁻,

where an increase in concentration of one species in equilibria will change the concentration of all species so that the equilibrium product maintains the equilibrium constant. Thus, in the system, the equilibrium concentrations of H₂CO₃, HCO₃ ⁻, and CO₃ ²⁻vs. pH in the electrolyte will follow the carbonate speciation diagram as discussed above.

In the system as illustrated in FIG. 1 and as discussed with reference to the carbonate speciation graph, supra, the solubility of carbon dioxide in the cathode electrolyte is dependent on the pH of the electrolyte. Also in the system, the voltage across the cathode and anode is dependent on several factors including the pH difference between the anode electrolyte and cathode electrolyte. Thus, in some embodiments the system can be configured to operate at a specified pH and voltage to absorb carbon dioxide and produce carbonic acid, carbonate ions and/or bicarbonate ions in the cathode electrolyte. In embodiments where carbon dioxide gas is dissolved in the cathode electrolyte, as protons are removed from the cathode electrolyte more carbon dioxide may be dissolved to form carbonic acid, bicarbonate ions and/or carbonate ions. Depending on the pH of the cathode electrolyte the balance is shifted toward bicarbonate ions or toward carbonate ions, as is well understood in the art and as is illustrated in the carbonate speciation diagram, above. In these embodiments the pH of the cathode electrolyte solution may decrease, remain the same, or increase, depending on the rate of removal of protons compared to rate of introduction of carbon dioxide. It will be appreciated that no carbonic acid, hydroxide ions, carbonate ions or bicarbonate ions are formed in these embodiments, or that carbonic acid, hydroxide ions, carbonate ions, bicarbonate ions may not form during one period but form during another period.

In another embodiment, the present system and method are integrated with a carbonate and/or bicarbonate precipitation system (not illustrated) wherein a solution of divalent cations, when added to the present cathode electrolyte, causes formation of precipitates of divalent carbonate and/or bicarbonate compounds, e.g., calcium carbonate or magnesium carbonate and/or their bicarbonates. In various embodiments, the precipitated divalent carbonate and/or bicarbonate compounds may be utilized as building materials, e.g., cements and aggregates as described for example in commonly assigned U.S. patent application Ser. No. 12/126,776 filed on May 23, 2008, herein incorporated by reference in its entirety.

In an alternative embodiment, the present system and method are integrated with a mineral and/or material dissolution and recovery system (not illustrated) wherein the acidic anode electrolyte solution 104 or the basic cathode electrolyte 108 is utilized to dissolve calcium and/or magnesium-rich minerals e.g., serpentine or olivine, or waste materials, e.g., fly ash, red mud and the like, to form divalent cation solutions that may be utilized, e.g., to precipitate carbonates and/or bicarbonates as described herein. In various embodiments, the precipitated divalent carbonate and/or bicarbonate compounds may be utilized as building materials, e.g., cements and aggregates as described for example in commonly assigned U.S. patent application Ser. No. 12/126,776 filed on May 23, 2008, herein incorporated by reference in its entirety.

In an alternative embodiment, the present system and method are integrated with an industrial waste gas treatment system (not illustrated) for sequestering carbon dioxide and other constituents of industrial waste gases, e.g., sulfur gases, nitrogen oxide gases, metal and particulates, wherein by contacting the flue gas with a solution comprising divalent cations and the present cathode electrolyte comprising hydroxide, bicarbonate and/or carbonate ions, divalent cation carbonates and/or bicarbonates are precipitated as described in commonly assigned U.S. patent application Ser. No. 12/344,019 filed on Dec. 24, 2008, herein incorporated by reference in its entirety. The precipitates, comprising, e.g., calcium and/or magnesium carbonates and bicarbonates in various embodiments may be utilized as building materials, e.g., as cements and aggregates, as described in commonly assigned U.S. patent application Ser. No. 12/126,776 filed on May 23, 2008, herein incorporated by reference in its entirety.

In another embodiment, the present system and method are integrated with an aqueous desalination system (not illustrated) wherein the partially desalinated water of the third electrolyte of the present system is used as feed-water for the desalination system, as described in commonly assigned U.S. patent application Ser. No. 12/163,205 filed on Jun. 27, 2008, herein incorporated by reference in its entirety.

In an alternative embodiment, the present system and method are integrated with a carbonate and/or bicarbonate solution disposal system (not illustrated) wherein, rather than producing precipitates by contacting a solution of divalent cations with the first electrolyte solution to form precipitates, the system produces a solution, slurry or suspension comprising carbonates and/or bicarbonates. In various embodiments, the solution, slurry or suspension is disposed of in a location where it is held stable for an extended periods of time, e.g., the solution/slurry/suspension is disposed in an ocean at a depth where the temperature and pressure are sufficient to keep the slurry stable indefinitely, as described in U.S. patent application Ser. No. 12/344,019 filed on Dec. 24, 2008, herein incorporated by reference in its entirety; or in a subterranean site. 

1. An electrochemical system comprising: a gas diffusion anode; and a cathode contacting a cathode electrolyte comprising dissolved carbon dioxide.
 2. The system of claim 1, further comprising a gas delivery system configured to deliver hydrogen gas to the anode.
 3. The system of claim 2, wherein the hydrogen gas is produced at the cathode.
 4. The system of claim 2, wherein the anode is configured to produce protons, and the cathode is configured to produce hydrogen gas and hydroxide ions on application of a voltage across the anode and the cathode.
 5. The system of claim 4, wherein the voltage is less than 2V.
 6. The system of claim 4, wherein the system is configured not to produce a gas at the anode.
 7. The system of claim 4, further comprising a first cation exchange membrane positioned between the cathode electrolyte and a salt solution, and an anion exchange membrane positioned between the salt solution and an anode electrolyte.
 8. The system of claim 7, wherein the anode contacts the anode electrolyte.
 9. The system of claim 7, wherein a second cation exchange membrane is positioned between the anode and the anode electrolyte.
 10. The system of claim 9, wherein the system is configured to migrate anions to the anode electrolyte from the salt solution through the anion exchange membrane.
 11. The system of claim 9, wherein the system is configured to migrate chloride ions to the anode electrolyte from the salt solution through the anion exchange membrane.
 12. The system of claim 11, wherein the system is configured to migrate cations to the cathode electrolyte from the salt solution through the first cation exchange membrane.
 13. The system of claim 12, wherein the system is configured to migrate sodium ions to the cathode electrolyte from the salt solution through the first cation exchange membrane.
 14. The system of claim 9, wherein the system is configured to migrate protons to the anode electrolyte from the anode.
 15. The system of claim 14, wherein the system is configured to migrate hydroxide ions to the cathode electrolyte from the cathode.
 16. The system of claim 15, wherein the system is configured to produce sodium hydroxide and/or sodium bicarbonate and/or sodium carbonate in the cathode electrolyte.
 17. The system of claim 14, wherein the system is configured to produce an acid in the anode electrolyte.
 18. The system of claim 17, wherein the system is configured to produce hydrochloric acid in the anode electrolyte.
 19. The system of claim 9, further comprising a partition that partitions the cathode electrolyte into a first cathode electrolyte portion and a second cathode electrolyte portion, wherein the second cathode electrolyte portion contacts the cathode and comprises dissolved carbon dioxide.
 20. The system of claim 19, wherein the first cathode electrolyte portion comprises gaseous carbon dioxide.
 21. The system of claim 19, wherein the partition is positioned to isolate gaseous carbon dioxide gas in the first cathode electrolyte portion from cathode electrolyte in the second cathode electrolyte portion.
 22. The system of claim 21, wherein the system is configured to produce hydroxide ions and hydrogen gas at the cathode.
 23. The system of claim 22, wherein the system is configured to produce hydroxide ions in the cathode electrolyte.
 24. The system of claim 23, wherein the system is configured to migrate cations to the cathode electrolyte through the first cation exchange membrane; migrate anions to the anode electrolyte through the anion exchange membrane; and migrate protons to the anode electrolyte from the anode.
 25. The system of claim 24, wherein the system is configured to produce cations, hydroxide ions and/or carbonic acid and/or carbonate ions and/or bicarbonate ions in the cathode electrolyte.
 26. The system of claim 25, wherein the system is configured to produce sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte.
 27. The system of claim 26, wherein the cathode electrolyte is operatively connected to a carbon dioxide gas/liquid contactor configured to dissolve carbon dioxide in the cathode electrolyte.
 28. The system of claim 27, wherein the cathode electrolyte is operatively connected to a system configured to produce carbonates and/or bicarbonates and/or hydroxides from a solution comprising carbon dioxide and divalent cations.
 29. An electrochemical method comprising: applying a voltage across a cathode and a gas diffusion anode in an electrochemical system, wherein the cathode contacts a cathode electrolyte comprising dissolved carbon dioxide.
 30. The method of claim 29, comprising oxidizing hydrogen gas at the anode.
 31. The method of claim 30, comprising producing protons at the anode.
 32. The method of claim 30, comprising producing hydroxide ions and hydrogen gas at the cathode.
 33. The method of claim 32, wherein a gas is not produced at the anode.
 34. The method of claim 32, wherein the voltage is less than 2V.
 35. The method of claim 32, comprising directing hydrogen gas from the cathode to the anode.
 36. The method of claim 31, comprising migrating protons from the anode to an anode electrolyte.
 37. The method of claim 34, comprising interposing a cation exchange membrane between the anode and an anode electrolyte.
 38. The method of claim 37, comprising interposing an anion exchange membrane between the anode electrolyte and a salt solution.
 39. The method of claim 38, comprising interposing a first cation exchange membrane between the cathode electrolyte and the salt solution, and wherein the salt solution is disposed between the anion exchange membrane and the first cation exchange membrane.
 40. The method of claim 39, comprising migrating anions from the salt solution to the anode electrolyte through the anion exchange membrane, and migrating cations from the salt solution to the cathode electrolyte through the first cation exchange membrane.
 41. The system of claim 40, comprising producing hydroxide ions and/or carbonate ions and/or bicarbonate ions in the cathode electrolyte; and an acid in the anode electrolyte.
 42. The method of claim 40, comprising producing sodium hydroxide and/or sodium carbonate and/or sodium bicarbonate in the cathode electrolyte; and hydrochloric acid in the anode electrolyte.
 43. The method of claim 42, comprising contacting the cathode electrolyte with a divalent cation solution comprising calcium and/or magnesium ions.
 44. The method of claim 42, comprising producing partially desalinated water in the salt solution. 